The present invention relates to a separating cyclone for separating a mixture of liquids and/or gases into a heavy fraction with one or more liquids and/or gases of a relatively high specific mass and a light fraction with one or more liquids and/or gases of a relatively low specific mass. The invention also relates to the separation of such a mixture of liquids and/or gases into a heavy fraction and a light fraction.
Such separating cyclones, also referred to as hydrocyclones, are used mainly, though not exclusively, to separate oil and water in the oil industry. The light fraction, also referred to as the light phase, is formed here by the oil, while the heavy fraction, also referred to as the heavy phase, is formed by the water. In a known type of hydrocyclone a mixture of liquids and/or gases is guided via a tangential inlet into the cyclone. As a result of the tangential inlet the incoming mixture is set into rotation. Because of the difference in density between the light fraction and heavy fraction the light phase will be displaced to a central region in the middle of the cyclone, while the heavy fraction is displaced under the influence of centrifugal forces to a peripheral region close to the inner surface of the cyclone tube. A flow body provided with a discharge channel (also referred to as “vortex finder”) can be placed in the middle of the cyclone.
In a reverse flow hydrocyclone the light fraction changes direction and is discharged via said discharge channel in the direction of the light phase outflow, and subsequently discharged from the outflow. The side from which the light phase is discharged is also referred to as the overflow or reject side. The heavy phase leaves the cyclone tube on the side opposite the inlet, this side also being referred to as the underflow side. It is the desire in principle that only the light phase leaves the cyclone on the overflow side. It is the case in practice that, in order to achieve stable operation of the cyclone, an unnecessarily large quantity of heavy fraction also leaves the cyclone on the overflow side in addition to the light fraction.
In cyclones of another type (also referred to as axial flow hydrocyclones) the incoming mixture is supplied axially instead of tangentially, and the mixture is set into rotation by a swirl element. The swirl element comprises one or more stationary curved guide fins which set the mixture flowing along them into rotation. Such an axial flow hydrocyclone has the advantage that there is a lower pressure drop and a more uniformly rotating flow pattern over the cyclone, whereby a more stable interface can be realized between the light fraction and the heavy fraction. Axial flow hydrocyclones can moreover be constructed with a more limited overall length.
In both types of cyclone there are provided valves on the overflow side and the underflow side with which the pressure on each of the sides can be controlled. By adjusting the valves the position of the interface between the above-mentioned central region, in which the light fraction is situated, and the peripheral region, in which the heavy fraction is located, can be adjusted as required.
A drawback of both types of hydrocyclone is however that it has been found that, in order to be able to bring about a reasonable separation of the light fraction (usually the oil), 20 to 50 times the volume of the light fraction of heavy fraction must leave the overflow outlet. In axial cyclones the volume percentage of the light fraction relative to the heavy fraction is for instance about 2% and in tangential cyclones this proportion is about 4%. This means that the separated light fraction remains to large extent mixed with the heavy fraction, which is of course undesirable in terms of separating efficiency.
From the document U.S. Pat. No. 6,024,874 a cyclone is known which is provided with a tangential inlet for setting an incoming mixture into rotation. This brings about a separation of the mixture into a light and a heavy fraction. One fraction is discharged via a discharge channel and outlet provided in a flow body arranged centrally in the cyclone, while the other fraction is discharged via an opposite outlet. The discharge channel is provided with a channel portion with a cross-section decreasing in the flow direction. The known cyclone does not however have valves with which the pressure on the side of the light fraction discharge and on the side of the heavy fraction discharge can be adjusted, and therewith the position of the interface between the region of the light fraction and the region of the heavy fraction. The separation by the known cyclone is hereby less stable and the separating efficiency is limited.
It is an object of the present invention to provide a separating cyclone, method and assembly with which a higher separating efficiency can be achieved.
Is also an object of the present invention to provide a separating cyclone, method and assembly with which, during separation of a light phase from a mixture of liquids and/or gases, the required quantity of heavy fraction entrained with the light fraction is reduced.
It is also an object of the invention to provide a separating cyclone, method and assembly in which separation can be performed in more stable manner.